Practically all modern electronic equipment has yielded to the incorporation of microprocessors to improve functionality and to reduce cost. Most electro-mechanical devices can be built using special purpose hardware such as transducers, switches, and motors that are turned on and off; plus software that tells the hardware what to do under various conditions. A microprocessor operates as an interface that controls the hardware in accordance with stored software instructions. It is important that such microprocessor-controlled devices operate properly over a broad range of environmental conditions such as wide temperature extremes, particularly in the case of a coin chute which must demonstrate high reliability because many persons become emotional when parting with their money, particularly when they receive nothing in return.
Mechanical coin chutes have been used for years in vending machines, public telephones and the like. Not only are such coin chutes bulky and expensive, they account for at least 50% of the problems associated with the equipment to which they are attached. Recently, electronic means have been used to simplify coin chute design, improve its reliability, and reduce its cost. However, electronic coin chutes (ECCs) have not been without problems such as accuracy of coin identification, and operation with a limited amount of electrical power. Keeping prices competitive with the mechanical designs that have been around for years was quite challenging initially. However, price reductions of microprocessors and associated memory devices have made lower cost and improved functionality a routine matter.
Nevertheless, reliability of identification for a wide variety of coins still presents a challenge for designers, particularly in those parts of a country where similar foreign coins of lesser denomination are readily available. This challenge is particularly difficult when accuracy over a broad temperature range is needed such as in the case of outdoor vending machines and public telephones. Coin quality sensing circuits can be specifically designed to be insensitive to temperature change; however, in view of the high accuracy requirements needed for coin handling, these circuits tend to be expensive and only compensate a portion of the temperature range.
The time that a coin remains within the coin path of an ECC is minimal because the coin path is typically free from obstructions. Indeed, most ECCs have only one moving part--the coin diverter--which is used to either return a coin to the depositor or divert it into a collection box. This decision must be made after the final quality sensor has examined the coin, and in sufficient time to operate the mechanical coin diverter. Such decisions normally require a microprocessor having great speed which leads to high cost and increased power consumption.
U.S. Pat. No. 3,198,564 discloses a technique in which a comparison is made between a measured value (such as frequency) of a coin quality sensor when a coin is in its presence, and when a coin is not. These values are examined and a signal (such as their arithmetic difference) is transmitted to a comparison and memory circuit. The comparison and memory circuit contains information regarding values for valid coins, and means for comparing such values with the transmitted signal. This approach assumes that the difference in characteristics remains constant with temperature, which it does not. Further, should the information regarding values for valid coins include a temperature look-up table for each of the various allowable coins, then the required memory space and microprocessor speed required to carry out the necessary calculations could be prohibitive in view of (i) cost, (ii) time available to perform calculations before an accept/reject decision on a coin must be made, and (iii) limited electrical power available in a line-powered public telephone application.